Project Summary/Abstract The prevalence of obesity in the United States is projected to rise to nearly 50% and cost over $400 billion by the year 2030. Despite the staggering economic burden, the underlying physiological mechanisms contributing to obesity are largely unknown. Appetitive and consummatory behaviors, thought to be dysregulated in obesity, are governed by descending signals from the brain, but how specific neural circuits regulate such behaviors is unclear. One critical component in the neural circuitry that orchestrates feeding behavior is the lateral hypothalamic area (LHA), a molecularly and functionally heterogeneous hypothalamic area that is interconnected with limbic and hindbrain structures. Ablating LHA abolishes feeding and disrupts body weight regulation across species, while electrical stimulation of LHA potentiates appetitive behavior and promotes food intake. However, such coarse manipulations have nonspecific effects on movement and motivation, which are likely due to the striking heterogeneity of the LHA. As a result, the function of discrete cell types within the LHA their contributions to feeding, energy homeostasis, and ultimately obesity remain elusive. Recent technological developments have made it possible to acutely manipulate and monitor activity dynamics of molecularly and anatomically defined neuron populations in behaving rodents. The goal of the proposed research is to use contemporary circuit neuroscience tools to monitor and manipulate the activity of molecularly- and anatomically-defined LHA neurons to determine how divergent projection pathways uniquely contribute to obesity. This project will focus on glutamatergic output pathways from the LHA to the ventral tegmental area or the lateral habenula, two brain regions that are known to regulate feeding and reward. Activity dynamics of these neurons will be monitored using in vivo two-photon deep brain calcium imaging coupled with optogenetics and electrophysiology throughout the onset of diet-induced obesity. Completion of the proposed aims is expected to be impactful because these studies will illuminate the causal and natural neural dynamics that underlie the onset of obesity. While neurons of the lateral hypothalamic area have long been implicated in the regulation of food intake and body weight, the genetic, molecular, and circuit mechanisms underlying these influences are unknown. Identifying how obesity interacts with defined neural circuitry at the level of individual neurons is of critical importance because without this information, we are unlikely to discover the ways in which obesity arises. This career development award will provide technical and conceptual training as well as mentorship from renowned experts in the field of circuit neuroscience. Ultimately, this training will uniquely position this young investigator to transition to an independent research program focused on investigating the neuronal basis of obesity.